There has been an industry trend from analog to digital modulation in communication systems. Some of the benefits of digital modulation include more robust communication, ability to introduce security through encryption/de-encryption, the ability to multiplex multiple forms of data (e.g., data, voice, video . . . ) and lower implementation costs, among others.
Digital modulation can be used to transfer digital serial data over an RF passband waveform. Digital modulation techniques include, but are not limited to, quadrature phase shift keying (QPSK), frequency shift keying (FSK), minimum shift keying (MSK) and quadrature amplitude modulation (QAM). Digital modulation is used in wireless and wired communication systems. Cellular, satellite, terrestrial and broadband cable systems represent communication systems implementing digital modulation.
A digital to analog convertor (DAC) or an equivalent channel generation device is used to convert serial digital data for transmission in communication systems with an RF passband waveform. The serial digital data is sampled and converted to an intermediate frequency (IF) waveform at a rate identified based on the Nyquist theorem. A byproduct of the sampling process and the physical implementation is often the generation of in-band and/or out-of-band noise in the RF waveform. Noise, spurious emissions, harmonics and other characteristics may be introduced when the generation of the RF passband waveform does not meet the RF specifications of the communication system. These effects can result in limiting the utilization of any channel in the RF passband, and/or additional circuitry may be introduced to meet the RF specifications of the communication system.
Like reference numbers and designations in the various drawings indicate like elements.